tests: workq/user_work: change how resource pool is exhausted

The assumption of going through 100 iterations of allocating for
work queue is that the HEAP size is 1024 bytes. However, there
are platforms where the HEAP size is inflated due to subsystems
and/or drivers being included. So change the for loop into
a loop where it only stops until there is no memory to be
allocated.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>